The most useful user-made C-macros (in GCC, also C99)? [closed]

Answer 1

Pack bytes,words,dwords into words,dwords and qwords:

#define ULONGLONG unsigned __int64
#define MAKEWORD(h,l) ((unsigned short) ((h) << 8)) | (l)
#define MAKEDWORD(h,l) ((DWORD) ((h) << 16)) | (l)
#define MAKEQWORD(h,l) ((ULONGLONG)((h) << 32)) | (l) 

Parenthesizing arguments it's always a good practice to avoid side-effects on expansion.